1. Field of the Invention
This invention relates generally to a method and apparatus for controlling an internal combustion engine and specifically to a method and apparatus for maintaining a desired combustion parameter in the engine even though there may be degradation of the engine or its controls.
2. Discussion of the Prior Art
In view of the increasing stringency of emission control regulations in various countries in recent years, many attempts have been made to improve fuel supply systems of engines to reduce noxious exhaust emissions while maintaining good engine drivability.
One well-known approach to controlling ignition timing and/or fuel injection in an internal combustion engine makes use of digital "maps" i.e. memories pre-programmed with data relating to ignition timing, fuel injection, exhaust gas recirculation (EGR), etc. for each of a multiplicity of combinations of values of two or more different engine parameters such as throttle valve angle, engine speed, manifold pressure, etc. While this approach has provided a great improvement in ignition and fueling efficiency (since such maps represent very complex surfaces unobtainable using mechanical cams or simple electronic function generators) they do not provide a completely adequate answer to emission and efficiency problems since there are many variables which cannot easily be taken into account. Such variables include fuel composition, partial fouling of fuel injectors, the effect of deposits on the engine cylinder head, ignition energy and spark gap variations, and cylinder to cylinder variability.
Attempts have been made to overcome these shortcomings by the introduction of closed loop controls in which an "output" of the engine is detected and a corresponding signal is fed back into a control system to adjust an "input" to the engine. Thus, for example, an oxygen sensor in the exhaust system can be used to adjust fueling and this will take care of variations in fuel composition and air/fuel ratio, but will not necessarily make correct adjustments in response to changes in other variables, such as exhaust gas recirculation (EGR) and compression ratio.
To cope with these problems, various sophisticated sensors have been designed to measure, for example, cylinder pressure or flame front ionization, but the use of these involves high cost, possible unreliability and some compromise in cylinder head geometry.
Closed loop mixture control systems have been proposed which detect the magnitude of the variation of engine output in successive combustion cycles. However, these methods only have good sensitivity with mixtures having air/fuel ratios close to the lean-running limit, or with mixtures having a large percentage of EGR.
It has also been proposed (see SAE paper No. 780655 "Electronic Spark Timing Control for Motor Vehicles" by Paul H. Schweitzer and Thomas W. Collins) to optimize ignition timing by means of a closed loop system in which a "dither" or periodic variation is superimposed on the ignition timing. The result of the dither is analyzed to determine the effect on engine output with respect to variations in ignition timing. The basic timing on which the dither is superimposed is adjusted to give a maximum of some engine output such as torque or speed. Thus, the basic ignition timing is adjusted so that a change in timing in either direction results in a reduction of engine torque or speed (since the timing would be moving away from the optimum).
It has been found, however, that optimizing spark timing in this way does not provide a complete answer, since it cannot correct errors which result from the fueling of the engine. With optimum fueling, optimizing of spark timing can certainly improve the emission performance, but if fueling is less than optimum, more is required.